This invention relates generally to optical waveguide devices and, more particularly, to devices for multiplexing two orthogonally polarized optical signals, or for demultiplexing a composite signal into two orthogonally polarized signals. Electrooptical devices are preferably operated using optical signals that are linearly polarized in one direction, such as in the transverse magnetic (TM) mode. In optical communications systems, there is frequently a need to combine two orthogonally polarized optical signals in a single waveguide, or to separate the two polarization modes, the TM mode and the transverse electric (TE) mode into two waveguides. Devices for performing these functions are called TE/TM multiplexer/demultiplexers, or TE/TM splitter/combiners.
There are many publications relating to this general topic. The following are represenative papers describing prior TE/TM multiplexer/demultiplexers:
R. C. Alferness et al., "Low-cross-talk waveguide polarization multiplexer/demultiplexer," Optics Lett. Vol. 10, No. 4, April 1984, pp. 140-142.
Osamu Mikami, "LiNbO.sub.3 coupled-waveguided TE/TM mode splitter," Appl. Phys. Lett. 36(7), Apr. 1, 1980, pp. 491-493.
Hirochika Nakajima et al., "Crosstalk Characteristics of Ti-LiNbO.sub.3 Intersecting Waveguides and Their Application as TE/TM Mode Splitters," IEEE J. of Quant. Elec., Vol QE-18, No. 4, April 1982, pp. 771-75.
D. Yap et al., "Passive Ti-LiNbO.sub.3 channel waveguide TE-TM mode splitter," Appl. Phys. Lett. 44(6), Mar. 15, 1984, pp. 583-585.
All of the polarization multiplexers or power combiners of the prior art either employ electrooptical devices or intersecting waveguides. When intersecting waveguides are used, there is considerable cross-talk between the two polarization modes, and precise control must be exercised over the fabrication conditions. If electrooptical effects are employed, it is also difficult to obtain low cross-talk components simultaneously for both polarization modes. Another approach is to employ a metallic overlay to force a mismatch between the modes of the waveguide. However, this requires a more complex fabrication technique.
Accordingly, there is still a need for improvement in the field of TE/TM multiplexer/demultiplexers and the present invention satisfies this need.